Field of the Invention
The invention relates to the field of drag reduction in moving objects, particularly drag reduction in objects moving through fluids, and more particularly to methods and systems for reducing air drag on land-based vehicles moving on roadways.
Background of the Art
The wake formed behind a moving body, particularly a road-based vehicle, can be many multiples of the vehicle length. The cross-sectional area of the wake also generally expands well beyond that of the vehicle. The wake is a complex, time dependent fluid flow phenomena that has the general effect of producing a low pressure area following the vehicle. It contains both coherent and non-coherent vortex structures that contribute significantly to the building and expansion of the wake region behind the vehicle. The difference between the high pressure generated at the front of the vehicle and the low pressure wake region directly behind the vehicle exerts a net force which is opposite to the direction of motion of the vehicle. The energy required to form and sustain the wake must come from the vehicle power source, therefore the wake has a significant negative impact on the fuel consumption rate of the vehicle.
The drag caused by wakes, currents and eddies adjacent to vehicles has significant impact on forces affecting the efficiency of vehicle movement and energy use on the vehicles. Much effort has been put into aerodynamic design on vehicles to reduce drag to increase fuel efficiencies. Drag, especially at speeds above 30 km/hour, becomes increasingly important in fuel efficiency and is still a major concern for vehicle manufactures.
Many drag reduction devices have been proposed in the technical and patent literature. The majority of the aerodynamic-alteration devices developed or proposed to date are fixed devices that actually interfere with normal operation of the trailer, particularly loading and unloading operations. Because they are fixed, previously proposed devices tended to function adequately only under optimal conditions; their effectiveness is negated by changing environmental conditions and road geometry. These devices included streamlining or boat tail add-on devices, fixed plate extensions at the rear of the vehicle, suction and blowing devices, flaps and fairings, underbody skirts and wedges, tractor/trailer gap flow control devices and, more recently, flapping devices to add energy content to the wake region at a specific frequency.
Many of these devices have been evaluated and tested by government laboratories and trucking associations. Considerable work, including a significant amount funded by the U.S. Department of Energy (DOE), has been performed on creating and evaluating drag reduction technologies for large trucks. One such major study was the DOE Project on Heavy Vehicle Aerodynamic Drag, which was conducted over a three year period. An on-road test of drag reduction concepts has been completed by the Truck Manufacturers Association in conjunction with the DOE. A significant number of patents cover a wide range of concepts developed to date. However, none of these technologies have demonstrated the actual capability to achieve the benefits sought.